One obviously has to be reasonably fit to hike anywhere 5-18 hours per day for 8 days; it is altitude sickness which most often forces people to turn back. I will be crushed if I have to do that.
The highest I've ever been is Dead Woman's Pass on the Inca Trail: 4,200 metres. I felt fine at that. We pass that level before the end of day 3. Kilimanjaro's highest peak is 5895 metres.
I've been reading a lot about Diamox and have decided to take it in preventative measures rather than if I begin to experience any uncomfortable symptons. There's a cut-off period where if you haven't taken it already, you can't start so it seems a bit risky to me. Apparently it makes you pee more. As I already have to get up twice in the night, I am seriously considering a women's pee funnel!
